Device manufacturing method, computer program and lithographic apparatus

ABSTRACT

In a device manufacturing method using a lithographic apparatus, corrections to the dose are applied, within and/or between fields, to compensate for CD variations due to heating of elements of the projection system of the lithographic apparatus.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of co-pending U.S. application Ser.No. 11/713,787 filed on Mar. 5, 2007 by VAN DE KERKHOF, Marcus Adrianuset al., entitled DEVICE MANUFACTURING METHOD, COMPUTER PROGRAM ANDLITHOGRAPHIC APPARATUS, the entire contents of which is herebyincorporated by reference and for which priority is claimed under 37U.S.C. §120.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lithographic apparatus and a methodfor manufacturing a device.

2. Background Art

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.including part of one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at once, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

It is well-known in the art of projection lithography that duringexposures, elements in the projections system absorb radiation, heat-upand therefore introduce aberrations into the projections system,resulting in reduced image quality at substrate level. These effects areparticularly acute when using illumination modes, such as dipoleillumination and quadrupole illumination, in which the intensitydistribution in the pupil plane of the illumination system and/or deepultraviolet radiation (DDV), e.g. at 198, 157 or 126 nm, since thechoice of materials from which lenses useable with these wavelengths canbe made is quite limited and even the best materials have significantcoefficients of absorption at those wavelengths. The problemparticularly affects projection systems formed by refractive lenselements and is therefore often referred to as lens heating. Even withcooling systems to maintain the projection system at a constanttemperature, sufficient local temperature variations can occur to causenoticeable loss of imaging quality.

Therefore, many projection systems in lithographic projection apparatusare provided with one or more actuated adjustable elements whose shape,position and/or orientation in one or more degrees of freedom can beadjusted during or between exposures to compensate for lens heatingeffects. A computer model predicts the lens heating effects that areexpected and caICulates appropriate corrections to be effected by theadjustable elements. Prior art computer models have caICulated the lensheating effects in terms of Zemike polynomials describing theaberrations in the pupil plane of the projection system and appliedcorrections via control “knobs” on the projection system that adjust oneor more adjustable elements to give a correction corresponding to therelevant Zemike polynomial. However, prior art lens heating correctionmethods have not always been completely effective and some residualaberrations often occur.

Other attempts to deal with the problem of non-uniform lens heatinginclude the provision of additional light sources, e.g. infra-red, toheat the “cold” parts, i.e. those not traversed by the intense parts ofthe projection beam, of elements of the projection system, see U.S. Pat.No. 6,504,597 and JP-A-08-221261. The former reference addressesnon-uniform heating caused by a slit-shaped illumination field and thelatter references addresses non-uniform heating caused by zonal ormodified illumination. The provision of such additional light sourcesand guides to conduct the additional heating radiation to the correctplaces may increase the complexity of the apparatus and the increasedheat load in the projection system necessitates the provision of acooling system of higher capacity.

BRIEF SUMMARY OF THE INVENTION

It is desirable to provide an improved method for at least reducing ormitigating the effects of non-uniform heating of elements of aprojection system.

According to an embodiment of the invention, there is provided a devicemanufacturing method in which an image of a given pattern includingfeatures having a critical dimension is projected onto a substrate usinga projection system having at least one optical element that issensitive to heat, the method including determining variations in thecritical dimension of the features that would be expected to occur whenprojecting the image at a nominal dose due to heating of the opticalelement; determining dose corrections to vary the critical dimension ofthe features to at least partially compensate for the expectedvariations in the critical dimension; and projecting an image of thepattern onto the substrate whilst applying the determined dosecorrections.

According to an embodiment of the invention, there is provided acomputer program product including instructions recorded on acomputer-readable medium, the instructions being such as to control alithographic apparatus, having a projection system having at least oneoptical element that is sensitive to heat, to perform a devicemanufacturing method in which an image of a given pattern includingfeatures having a critical dimension is projected onto a substrate, themethod including determining variations in the critical dimension of thefeatures that would be expected to occur when projecting the image at anominal dose due to heating of the optical element; determining dosecorrections to vary the critical dimension of the features to at leastpartially compensate for the expected variations in the criticaldimension; and projecting an image of the pattern onto the substratewhilst applying the determined dose corrections.

According to an embodiment of the invention, there is provided alithographic apparatus including: an illumination system configured tocondition a radiation beam so as to illuminate a patterning device; asupport constructed to support the patterning device, the patterningdevice being capable of imparting the radiation beam with a pattern inits cross-section to form a patterned radiation beam; a substrate tableconstructed to hold a substrate; a projection system configured toproject the patterned radiation beam onto a target portion of thesubstrate, the projection system having at least one optical elementthat is sensitive to heat; and a control system configured to controlthe illumination system, the support and the substrate table to projectimages of the pattern onto a substrate using a corrected dose thatdiffers from a nominal dose by an amount determined to compensate forvariations in critical dimension of projected features due to heating ofthe optical element.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 depicts the optical arrangements of the apparatus of FIG. 1;

FIG. 3 depicts a device configured to vary the intensity oaf radiationbeam across the length of the illumination slit according to anembodiment of the invention;

FIG. 4 depicts a device configured to vary the dose delivered atsubstrate level across the length of the illumination slit according toan embodiment of the invention; and

FIG. 5 depicts a device manufacturing method according to an embodimentof the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus includes an illuminationsystem (illuminator) IL configured to condition a radiation beam B (e.g.UV radiation or DUV radiation); a support structure (e.g. a mask table)MT constructed to support a patterning device (e.g. a mask) MA andconnected to a first positioner PM configured to accurately position thepatterning device in accordance with certain parameters; a substratetable (e.g. a wafer table) WT constructed to hold a substrate (e.g. aresist coated wafer) Wand connected to a second positioner PW configuredto accurately position the substrate in accordance with certainparameters; and a projection system (e.g. a refractive projection lenssystem) PS configured to project a pattern imparted to the radiationbeam B by patterning device MA onto a target portion C (e.g. includingone or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports, i.e. bears the weight of, the patterningdevice. It holds the patterning device in a manner that depends on theorientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning device is held in a vacuum environment. The support structurecan use mechanical, vacuum, electrostatic or other clamping techniquesto hold the patterning device. The support structure may be a frame or atable, for example, which may be fixed or movable as required. Thesupport structure may ensure that the patterning device is at a desiredposition, for example with respect to the projection system. Any use ofthe terms “reticle” or “mask” herein may be considered synonymous withthe more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable ICD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleoaf programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use oaf vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is oaf transmissive type (e.g. employinga transmissive mask). Alternatively, the apparatus may be oaf reflectivetype (e.g. employing a programmable mirror array oaf type as referred toabove, or employing a reflective mask).

The lithographic apparatus may be oaf type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be oaf type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the patterning device (e.g. mask) and the projection system.Immersion techniques are well known in the art for increasing thenumerical aperture of projection systems. The term “immersion” as usedherein does not mean that a structure, such as a substrate, must besubmerged in liquid, but rather only means that liquid is locatedbetween the projection system and the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid oaf beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as a-outer anda-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL mayinclude various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table) MT, andis patterned by the patterning device. Having traversed the patterningdevice (e.g. mask) MA, the radiation beam B passes through theprojection system PS, which focuses the beam onto a target portion C ofthe substrate W. With the aid of the second positioner PW and positionsensor IF (e.g. an interferometric device, linear encoder or capacitivesensor), the substrate table WT can be moved accurately, e.g. so as toposition different target portions C in the path of the radiation beamB. Similarly, the first positioner PM and another position sensor (whichis not explicitly depicted in FIG. 1) can be used to accurately positionthe patterning device (e.g. mask) MA with respect to the path of theradiation beam B, e.g. after mechanical retrieval from a mask library,or during a scan. In general, movement of the support structure (e.g.mask table) MT may be realized with the aid of a long-stroke module(coarse positioning) and a short-stroke module (fine positioning), whichform part of the first positioner PM. Similarly, movement of thesubstrate table WT may be realized using a long-stroke module and ashort-stroke module, which form part of the second positioner PW. In thecase oaf stepper (as opposed to a scanner) the support structure (e.g.mask table) MT may be connected to a short-stroke actuator only, or maybe fixed. Patterning device (e.g. mask) MA and substrate W may bealigned using mask alignment marks M1, M2 and substrate alignment marksP1, P2. Although the substrate alignment marks as illustrated occupydedicated target portions, they may be located in spaces between targetportions (these are known as scribe-lane alignment marks). Similarly, insituations in which more than one die is provided on the patterningdevice (e.g. mask) MA, the mask alignment marks may be located betweenthe dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure (e.g. mask table) MT and thesubstrate table WT are kept essentially stationary, while an entirepattern imparted to the radiation beam is projected onto a targetportion C at one time (i.e. a single static exposure). The substratetable WT is then shifted in the X and/or Y direction so that a differenttarget portion C can be exposed. In step mode, the maximum size of theexposure field limits the size of the target portion C imaged in asingle static exposure.

2. In scan mode, the support structure (e.g. mask table) MT and thesubstrate table WT are scanned synchronously while a pattern imparted tothe radiation beam is projected onto a target portion C (i.e. a singledynamic exposure). The velocity and direction of the substrate table WTrelative to the support structure (e.g. mask table) MT may be determinedby the (de-)magnification and image reversal characteristics of theprojection system PS. In scan mode, the maximum size of the exposurefield limits the width (in the non-scanning direction) of the targetportion in a single dynamic exposure, whereas the length of the scanningmotion determines the height (in the scanning direction) of the targetportion.

3. In another mode, the support structure (e.g. mask table) MT is keptessentially stationary holding a programmable patterning device, and thesubstrate table WT is moved or scanned while a pattern imparted to theradiation beam is projected onto a target portion C. In this mode,generally a pulsed radiation source is employed and the programmablepatterning device is updated as required after each movement of thesubstrate table WT or in between successive radiation pulses during ascan. This mode of operation can be readily applied to masklesslithography that utilizes programmable patterning device, such as aprogrammable mirror array oaf type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 2 depicts the basic optical arrangement of the apparatus of FIG. 1.It uses Koehler illumination whereby a pupil plane PPj in theillumination system IL is a Fourier transform plane of the object planein which the patterning device MA is located and is conjugate to a pupilplane PPp (the projection system may have more than one pupil plane) ofthe projection system PS. As is conventional, illumination modes of thisapparatus can be described by reference to the distribution of intensityof the radiation of the projection beam in the pupil plane PPj of theillumination system. It will be understood that the distribution ofintensity in the pupil plane PPp of the projection system PS will be thesame as. the distribution of intensity in the pupil plane PPj of theillumination system, subject to diffraction effects of the patternpresent in patterning device MA.

For a pattern consisting essentially of lines in one direction, goodimaging and a large process window can be obtained by use of dipoleillumination in which the poles are arranged such that in the pupilplane PPp of the projection system, one of the first order diffractedbeams deriving from each of the two poles in the illumination systemcoincides with the zeroth order beam deriving from the other pole. Theother first order beams and higher order beams are not captured by theprojection system. It will thus be appreciated that the energy of theprojection beam is strongly localized in the projection system PS, atleast in elements close to the pupil plane(s). This localization of theintensity causes localized heating, which often changes over time, ofelements LE of the projection system which may give rise to aberrationsin the projection system PS that are not correctable by known adjustablelens elements AE.

In many cases, lens heating causes changes in the position of the planeof best focus relative to its nominal position, leading to focus errors,commonly termed defocus, in the projected image. These errors may varyacross a field, between fields on a single substrate and betweensubstrates in a single batch or lot. Variations between fields andbetween substrates may be caused by variation in the lens heatingeffects over time; variations within a field may be inherent in the lensheating effects themselves (i.e. changes in the shape of the plane ofbest focus rather than merely its position) and/or due to the patternbeing projected.

Within a certain range, defocus manifests in the printed image as avariation in the dimensions of the printed features, in particular thoseprinted at the critical dimension (CD) which are generally the mostimportant features in a pattern. According to an embodiment of thepresent invention, it is proposed to vary the dose delivered to thesubstrate to compensate for lens heating effects. Within a certainrange, similar to the range over which defocus manifests as a change inCD, changes in dose cause changes in CD. The exact relationship betweendose and CD is, in general dependent on parameters of the pattern beingexposed, in particular pitch. Thus CD variation caused by lens heatingeffects can be corrected for by compensatory changes in dose. Thisapproach contrasts with prior art approaches to dealing with lensheating, which have involved either thermal control of the projectionsystem—e.g. cooling systems and additional heating to reduce temperaturegradients in affected elements—or optical corrections—e.g. usingadjustable optical elements to introduce compensatory aberrations.

Dose at substrate level can be controlled in a number of ways. In astepper, dose can be controlled across the whole field by controllingthe output of the radiation source, using a variable attenuator VAT orchanging the exposure duration. In a scanner, or step-and-scanlithography apparatus, dose can also be controlled within the field. Inthe scan direction (e.g. Y), dose variation can be effected by varyingthe output of the radiation source, the setting of a variableattenuator, the width of the illumination slit or the scan speed duringthe scan.

In the perpendicular direction (e.g. X) several devices are available toprovide a dose variation. One dose variation device 10, shown in FIG. 3,includes a neutral density filter 11 which has a non-uniform absorptionpattern. It is moved by actuator 12 relative to the illumination slit ISso as to give an intensity variation across the length of the slit. Therange of intensity variations that can be effected by this device islimited but by appropriate selection of the absorption pattern of thefilter 11, a useful, if not always perfect, correction for expected lensheating effects can be effected. Another dose variation device 10′ isshown in FIG. 4. This is known as a dynamically adjustable slit andincludes a plurality of fingers 13 which can be individually andcontrollably extended into the illumination slit from one or both sidesthereof by an actuation system 14. The fingers can be opaque orpartially absorbing, optionally with a transmission gradient in thedirection (Y) perpendicular to the length of the slit (X). The finenessof the correction achievable by this method is determined by the widthof the fingers 13 and so can be set as desired. A variation of thedynamically adjustable slit that is particularly useful for EUV useswires or vanes extending across the slit and whose position, orientationand/or effective width can be varied. In this variation, the wires orvanes are put at a position along the beam path so that they are out offocus on the patterning device.

FIG. 2 shows schematically the optical and control arrangements of thelithographic apparatus embodiment of FIG. 1 in accordance with anembodiment of the present invention. FIG. 5 shows a device manufacturingmethod according to an embodiment of the invention. First, pattern data,relating to a pattern to be imaged, and illumination mode data, relatingto an illumination mode to be used, are received S1 by a control unit CDwhich caICulates S2 the lens heating effects that are to be expected,which may vary over time during exposure of a substrate or batch ofsubstrates, e.g. by reference to a lens heating model (LH model).Optionally, the illumination mode to be used may be caICulated orselected by the control unit on the basis of the pattern data.

Next, the control unit CD caICulates S3 corrections to be applied tocompensate for the lens heating effects. These corrections may beapplied by any available adjustable lens elements AE the projectionsystem and by adjusting the dose, as described further below, takinginto account whatever other corrections are required to apply tocompensate for any other forms of error in the apparatus or the processto be carried out on the substrates. The division of corrections betweenadjustable elements AE and dose will depend on the capabilities of theadjustable elements AE and other corrections to be applied. It may bedesirable, for example, to effect primary corrections with adjustableelements AE and residual corrections with dose variations. Once theappropriate corrections are determined, the substrates are exposed S4with the appropriate dose. It should be noted that blocks S1 to S3 maybe performed off-line in advance of the exposure S4 to maximizethroughput and may be performed by a computer separate from thelithographic apparatus with the corrections being communicated to thelithographic apparatus by network or data carrier.

Application of the dose variations may be facilitated by OoseMapper™software available from ASML Netherlands B.V., of Veldhoven, TheNetherlands, which is configured to apply a desired dose profile toexposures of substrates. The dose variations caICulated according to anembodiment of the invention may be combined with other dose variationsrequired to compensate for other effects, for example external causes ofCO non-uniformity which may be measured by in-line CD metrology.

In a variation of the method, appropriate corrections can be derivedempirically. Appropriate test structures are added to the pattern whichis printed on a batch of substrates and their critical dimensionsmonitored for variation across each substrate and between substrates ofthe batch. A correlation between lens heating over time and CD variationcan then be determined and appropriate corrections to be applied by dosevariation can be derived.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (ICDs), thin film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to anyone orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form oafcomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A device manufacturing method in which an image oaf given patterncomprising features having a critical dimension is projected onto asubstrate using a projection system having at least one optical elementthat is sensitive to heat, the method comprising: (a) determiningvariations in the critical dimension of the features that would beexpected to occur when projecting the image at a nominal dose due toheating of the optical element; (b) determining dose corrections to varythe critical dimension of the features to at least partially compensatefor the expected variations in the critical dimension; and (c)projecting an image of the pattern onto the substrate while applying thedetermined dose corrections.
 2. A method according to claim 1, whereinan image of the pattern is projected onto a plurality of separate targetportions on the substrate and at least one dose correction is determinedfor each target portion.
 3. A method according to claim 1, wherein animage of the pattern is projected onto a plurality of separatesubstrates and at least one dose correction is determined for eachsubstrate.
 4. A method according to claim 1, wherein the dosecorrections include dose corrections that vary across the image.
 5. Amethod according to claim 1, wherein determining the variations in thecritical dimension comprises caICulating variations in the criticaldimension using a model of the effect of heat on the projection systemand information relating to the pattern to be imaged and/or anillumination mode to be used in the projecting.
 6. A method according toclaim 1, wherein the image is projected onto a plurality of substratesand further comprising: (a) measuring CD non-uniformity due to causesexternal to a lithographic apparatus that includes the projection systemin at least a first one of the plurality of substrates; (b) determiningsecond dose corrections to compensate of the measured CD non-uniformity;and (c) exposing at least a second one of the plurality of substratesusing the second dose corrections combined with the first mentioned dosecorrections.
 7. A method according to claim 1, wherein determining thevariations in the critical dimension comprises projecting the image ontoat least one substrate a plurality of times to form a plurality ofphysical features and measuring critical dimensions of the physicalfeatures.
 8. A computer program product comprising instructions recordedon a computer-readable medium, the instructions to control alithographic apparatus, having a projection system having at least oneoptical element that is sensitive to heat, to perform a devicemanufacturing method in which an image oaf given pattern comprisingfeatures having a critical dimension is projected onto a substrate, themethod comprising: (a) determining variations in the critical dimensionof the features that would be expected to occur when projecting theimage at a nominal dose due to heating of the optical element; (b)determining dose corrections to vary the critical dimension of thefeatures to at least partially compensate for the expected variations inthe critical dimension; and (c) projecting an image of the pattern ontothe substrate while applying the determined dose corrections.
 9. Alithographic apparatus comprising: (a) an illumination system configuredto condition a radiation beam so as to illuminate a patterning ‘device;a support constructed to support the patterning device, the patterningdevice being capable of imparting the radiation beam with a pattern inits cross-section to form a patterned radiation beam; (b) a substratetable constructed to hold a substrate; (c) a projection systemconfigured to project the patterned radiation beam onto a target portionof the substrate, the projection system having at least one opticalelement that is sensitive to heat; and (d) a control system configuredto control the illumination system, the support and the substrate tableto project images of the pattern onto a substrate using a corrected dosethat differs from a nominal dose by an amount determined to compensatefor variations in critical dimension of projected features due toheating of the optical element.